These studies seek the stereospecificity, regulation and structure of the folate-antifolate transport mechanism in L1210 leukemia cells, other murine tumor cells and epithelial cell from mouse small intestine, an in vitro model for drug sensitive normal tissue. The influx and efflux and concentrative uptake of antifolates by both cell types will be quantitated in vitro and related to other intracellular parameters of therapeutic significance and to the uptake and loss from the same cells in vivo during therapy. Antifolate content of cells will be assayed by enzyme titration of extracts. The relevance of membrane permeability to selective antitumor activity by antifolate will be assessed and evaluated in respect to effects on DNA synthesis. The involvement of thymidylate synthetase in determining the magnitude of antitumor effects by pteridine and quinazoline antifolates will be examined by a kinetic analysis of inhibition and in vivio by the extent of reversibility of inhibition of DNA synthesis by leucovorin and other folate coenzymes. The relevance of membrane permeability to the reversal by normal folates of antifolate determined antileukemic activity and toxicity will be studied in vivo. The effect of normal folates on drug determined inhibition of DNA synthesis will be examined as a dose dependency in the animal. Epthelial cells from small intestine and unresponsive tumor cells with low antifolate transport capacity will be studied for the illicit uptake of antifolate via peptide transport mechanisms. The capacity and stereospecificity for peptide transport and the uptake of methotrexate peptide conjugates will be studied in vitro and in vivo. Therapy with methotrexate-peptides will be studied in animal tumor models. Therapy with antifolates that are not actively transported will be examined in conjunciton with leucovorin rescue.